The Bell Siphon. 

I want to discuss the construction, use and operation of the bell siphon or what some people call the auto siphon. The bell siphon is the secret to how an ebb and flow system works. The ebb and flow system allows the grow bed to be flooded, then drains the water down to the bottom. You can imagine a complicated array of timers, sensors, servos and valves to do this job, but really all you need is a water pump and a bell siphon. Here is a bad drawing of how it works.

We can see in figure 1a without a bell siphon, that the water is pumped up from the fish bed (not shown) into the grow bed. As the water fills the bed, it eventually becomes level with the standpipe and starts to drain back to the fish. the water flow is shown in red arrows. This method is great if we want to maintain an exact water level in the bed. The water will not get higher than the height of the standpipe. You can use this application in a fish bed that you do not want to overflow because of rain. As the rain fills your fish bed, the standpipe would keep the level the same as it was before it started raining. For our grow bed, however, we want the water to fill the bed then drain away almost completely. We want our roots soaked often with nutrient rich water from the fish bed, but we don't want them setting in water all the time. You can see from Figure 1b that with the bell siphon, water rises inside the siphon body until it reaches the standpipe height and then starts to drain. As the water drains, it builds up back pressure and creates a vacuum. Very quickly the vacuum outpaces the amount of water flowing into the system from the fish, and it drains the grow bed down to the bottom. Once air enters the siphon from the bottom, the suction is broken, and the bed starts to fill again. Over and over, it fills and drains, and no timers or controllers are needed. I hope that is a clear explanation on the operation of the bell siphon. Here is a video of one of mine working:

As you can see, the water level reaches a certain height and the pressure builds until the suction kicks in and then SLURP, all gone. To me it is just cool.

Now with a bell siphon it is all about water flow in vs. water flow out. It is a somewhat delicate balance. It really has to do with how much water is flowing into your system and the pipe sizes for draining that flow. You need enough water to flow over the standpipe to create the vacuum but it needs to be slow enough to let the bed almost completely drain and allow the vacuum to be broken by letting air enter the siphon. I will discuss how to manage the balance in the bell siphon build post next.

To build a bell siphon you need to answer a few questions first. Mostly you need to know how large of a grow bed are you going to fill and drain. This will determine what size pipes you will use. My experimental setup is very small but I use 1" PVC to drain it. At this point I want to link a document that helped me immensely. The paper is by the College of Tropical Agriculture and Human Resources in Hawaii and is about bell siphons. I am not sure about linking here but hopefully this does not violate any copyright stuff. Great paper and I used it over and over while building my first couple of siphons:

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/BIO-10.pdf

This paper describes the ratio between standpipe and bell siphon diameter. I found some very scientific analysis of the ratios in other published works, but prefer the method described in the paper above, where by you just double the diameter from standpipe to bell siphon to debris guard. So have used 1" for the standpipe, 2" for the bell siphon, and 4" PVC for the debris guard. 

To build the bell siphon you will need:

Materials:

2" PVC pipe

1" PVC pipe

2" PVC cap

100% silicone

Large zip tie

1/4" lead hose

PVC primer and cement

Tools:

Hacksaw

Dremel

Drill

1/4" drill bit

Pliers

Ruler

Pencil

To make the bell siphon, we are going to start with some measurements. We want the top of the standpipe to be below the rock or media in the grow bed. I found out the hard way that if your water comes over the grow media, and you get a nice sunny day, you will very efficiently grow algae, that will then be sucked down into the fish bed (see my earlier posts about my algae bloom). So I measure a rough estimate as to how full this grow bed will be and then figure out a height of the stand pipe. In this case I figured 6 inches. 

Next, I put the 2" PVC cap on the 2" inch piece of pipe and draw a circle under the lip of the cap. This is the guide as to where the top of the standpipe should end up. The paper by the College of Tropical Agriculture and Human Resources linked above states that the top of the standpipe must be flush with the bottom of the cap. This makes a bit of guessing as the cap slides down the 2" pipe quite a bit.

My solution for this calculation is to take a piece of 1" pipe and mark it the height that the standpipe will be, in this case 6". Now place the 2" pipe that will be the bell siphon body next to the 1" pipe and line the 1" pipe end up to the pencil ring that shows where the cap seated to. Mark the 2" pipe using the mark on the 1" pipe. This should give you the right length of bell siphon pipe to leave the standpipe at the same height as the bottom of the cap.

 Now cut the 2" pipe to size. I like a hacksaw for its accuracy and speed but you can use whatever you like. My first models I used a Dremel with the cutting wheel and just worked it around the pipe, but it always ended up uneven. Remember to use a medium grit sandpaper on the cut end to remove PVC shavings. I sand outside, then bottom edge then inside by wrapping the sandpaper around my finger.

Now you must make the intake ports on the bottom of the bell siphon. This is where water will fill the siphon at first until the pressure builds and then this is where water will be sucked through as the vacuum increases. There is some science as to size and flow rates, but I have just gone with 4 ports that are 1/2 inch by 1/2 inch. I use my Dremel to make 1/2 inch slices into the bottom of the pipe. You could use a hacksaw across the bottom as well. I just hack into the bottom with enough cuts so I can break the "teeth" out with my pliers.

Multiple cuts into the bottom of the bell siphon.

Snap the "teeth" out of the cuts to make your ports.

Next you will attach your snorkel. I have used 1/4" lead free hose. I bought a bit of it from Lowes as you can see from the photo. I was using this hose and some 1" lead free for a beer making project so I had some extra on hand. Find a drill bit that is a bit smaller than the hose. I find using a bit that is the same size as the  inside diameter of the hose works great as the outside diameter is larger and makes a good starting seal.

Drill a hole into the side of the 2" PVC cap and fit it to the 2" pipe. 

Give it a test to make sure the hose fits well in the hole you made.

Don't worry if the pipe and cap overlap in the hole, just drill again into the same hole and clear the pipe and hole. 

Once you are satisfied with the fit of everything, use the purple primer and PVC cement to put the cap on. You have to move quickly to line up the holes and remember, if you miss a little, no problem just gently re-drill the hole to remove any pipe that is blocking your way.

Here is you bell siphon so far.

Now you want to insert the hose into the hole you drilled. It only needs to go in a short way. Since you made the hole the same diameter as the inside of the hose, you will have to wiggle it a bit to get it in. After you have fitted the hose in place, seal it up with 100% silicone. The picture shows me using the silicone gun directly to the hose, but I found that if you put on some nitrile gloves and squirt some silicone on your finger, you can make this seal much neater and more effective.

I added a bead of silicone to the hose and wrapped some wire around it to hold it while it dried. This was no substitute for a zip tie around the snorkel, but it did offer some extra support.

Add a zip tie around the snorkel and the bell siphon to hold the snorkel in place and clip the snorkel hose. 

When you are going to snip the hose, remember that this will determine the height that your water will drain to. Once air enters the snorkel, the vacuum will be lost and the bell siphon will stop draining until it fills back up again. I have always cut my snorkel about an 1/8 of and inch above the intake ports. This means you will leave about 3/4 of an inch of water in your grow bed after it is drained. If you are using a shallow grow bed you may have to get more precise about the amount of water left in the system when it is in the drained state.

The next section will discuss adding the Grow bed to the table, and adding the piping. In that section we will talk about tuning the bell siphon to the water flow to get good consistent fills and drains. 

Please drop me a line if I can answer any questions about this process.